IntermediateTopic 68 of 106Electrical Engineering
Per-unit system
Normalises quantities to base values for easier power-system calculations.
In depth
Quantities are divided by base values: X_pu=actual/base. Transformer impedances are the same on both sides in pu, and three-phase systems simplify. Choose S_base and V_base, then Z_base=V²/S. Change base by Z_new=Z_old(S_new/S_old)(V_old/V_new)².
Key points
- Avoids voltage-level conversion
- Typical impedances 0.05–0.15 pu
- Used in fault and load-flow studies
ExampleA 10% reactance on 100 MVA is 0.1 pu.
What is Per-unit system?
Normalises quantities to base values for easier power-system calculations.
Why is it important to understand per-unit system?
The per-unit system makes power calculations easier, because quantities are normalised to base values and transformer ratios often disappear.
What is a base value?
A chosen reference for voltage, power, current and impedance. Each per-unit quantity is the actual value divided by its base.
Why does the per-unit system help with transformers?
Voltage bases can be chosen so the transformer ratio becomes one, so many transformer models simplify.
How is Per-unit system used in practice?
Typical applications include analysing a transformer in a power flow study without tracking turns ratios, checking the impedance of a generator on a common base and comparing the ratings of different equipment in a system model.
What should I learn before Per-unit system?
Study "Transmission lines" first (topic 67 of 106 in Electrical Engineering). It covers the groundwork this topic builds on.
What are the key points of Per-unit system?
Avoids voltage-level conversion. Typical impedances 0.05–0.15 pu. Used in fault and load-flow studies.
Can you give an example of Per-unit system?
A 10% reactance on 100 MVA is 0.1 pu.
What comes after Per-unit system?
The next topic is "Line parameters", which builds directly on this one.
How difficult is Per-unit system?
It is classed as Intermediate in Electrical Engineering (topic 68 of 106). It is an intermediate topic. Work through the beginner topics in Electrical Engineering first.